Nominating
Deoxys-Speed
What effect did Deoxys-Speed have on the metagame?
During the time that Deoxys-Speed was allowed in OU, it was the poster child of HO teams, thanks to its insane speed of 180 (so fast it rarely needed max speed) and its access to both Dual Screens and (much more importantly) Spikes and Stealth Rocks. Its insane speed, combined with its decent 95 SpA / Atk and great coverage made it a force to be reckoned with (it was decent enough that an offensive set was usable). Thanks to all these traits, Deoxys-S could always guarantee at least SR and maybe a layer of spikes on a bad day, and more often than not you have full hazards. For those who didn't play during the Deo-S meta, Deo-S plays very similar to Deo-D and Cutsap Skarm, except a lot better and a lot more efficient. Thanks to Deo-S, HO teams wrecked everything during its stay in OU.
What caused it to have a significant impact?
As stated above, Deo-S pretty much made HO a powerhouse of a playstyle. This was because it could always guarantee at least SR and possibly 1 spikes. Due to the fact that it could guarantee this, the rest of the team could easily destroy teams in a matter of turns. The Deo-S era was the golden age of HO during 5th Gen; hell, I'd argue it was the golden age of HO during the entirety of Pokemon. Like stated previously, it also had decent offenses, meaning it could kill or at the very least dent answers to Deo-S. For example, offensive Espeon gets 2HKO'd by Shadow Ball, meaning it has to play mindgames with Deo-S. Meanwhile, defensive sets have a chance to be 2HKO'd while not being able to do much back. Also, thanks to its amazing speed, Deo-S didn't need to invest in Speed, as it would naturally outspeed almost everything. This meant that it could either run mixed offenses or run HP investment to tank Scizor's Bullet Punch and set-up hazards (if Scizor U-Turns, Deo-S still gets hazards up). It also had some neat moves that it could use, such as Magic Coat for Sableyes and other Pranksters that try to get cute with Prankster Taunt, not to mention helping against other Deo-S.
How do/did you deal with this Pokemon in OU?
Deo-S was almost impossible to stop; If the foe had a Deo-S, you would have to deal with at least SR. Of course, it was possible to stop Deo-S from getting all of its hazads, but if it got SR and a later of Spikes, then it had done its job. Xatu is probably the best answer to Deo-S imo, since it doesn't get wrecked by Shadow Ball like Espeon does, but Xatu can't do much while it's on Deo-S except for Night Shade/Heat Wave (U-Turn wouldn't be a good idea) and Paralyze it (Toxic isn't useful against Deo-S). Prankser Taunt was a thing as well, but you needed to lead with it, and you needed to watch out for Magic Coat bouncing it back. Deo-S was also an option against Deo-S, since it could Taunt it or Magic Coat its hazards. Scarf Gengar was also an option, but Deo-S would at least get SR up if it had a Focus Sash. Although you could beat a certain combination of moves/items for Deo-S, you could not beat another set of Deo-S. For example, Scarf Gengar beats non-sash Deo-S, but Sash Deo-S gets SR against it. If Xatu gets hit by a Shadow Ball on the switch, it needs to pray that Deo-S isn't Lum or Magic Coats its T-Wave. Pranksters can beat non-Magic Coat Deo-S, but can't do much against Magic Coat Deo-S. So in conclusion, dealing with Deo-S was near impossible, since Deo-S could do so many things to bypass answers, meaning dealing with all of the Deo-S combinations was impossible.